The present invention relates to a plasma processing apparatus and a plasma processing method and, more particularly, to a plasma processing apparatus and a plasma processing method appropriate to perform high-accuracy etching processing using plasma in order to process a sample such as a semiconductor device.
Heretofore, as a method of processing the surface of a semiconductor device, there is known an apparatus of etching the semiconductor device with plasma. A related art is described here by taking a plasma etching apparatus with an electron cyclotron resonance (hereinafter, abbreviated to ECR) scheme as an example.
In the ECR scheme, plasma is generated by a microwave in a vacuum chamber to which a magnetic field is externally applied. Electrons undergo cyclotron motion by the magnetic field and the frequency thereof and the frequency of the microwave can be resonated to generate the plasma efficiently. In order to accelerate ions impinging on the semiconductor device, radiofrequency electric power of a substantially sinusoidal continuous wave is applied to the sample. The radio-frequency electric power applied to the sample is hereinafter referred to as a radio-frequency bias. Further, description is made by taking a wafer as an example of the sample.
Furthermore, halogen gas of chlorine, fluorine, or the like is widely used as a gas which becomes plasma. Radicals and/or ions generated by the plasma react with material to be etched, thereby advancing etching. In order to control the etching processing with high accuracy, it is necessary to perform selection of species of the radicals and control of the amount of ions by plasma control. As a control method of radicals and ions, there is a pulsed plasma scheme in which plasma is time-modulated. The pulsed plasma controls dissociation by repeating on and off of the plasma to control the dissociation state of radicals and/or the ion density. By taking a repetition frequency of the on and off of the pulsed plasma (hereinafter, referred to as a pulse frequency), a ratio of on-time to one period of the repetition frequency (hereinafter, referred to as a duty ratio), and/or a ratio of on-time and off-time as control parameters, high-accuracy control of the etching processing becomes possible. JP-A-59-047733 discloses an etching processing control method using pulsed plasma.
FIG. 1 shows a case where a radio-frequency bias of a continuous wave is applied to pulsed plasma. The radio-frequency bias is applied even during off-time of plasma. Generally, since the plasma density is low during off-time of plasma, an impedance viewed from the radio-frequency bias is increased and an amplitude value of the voltage applied to the wafer (hereinafter, referred to as Vpp) becomes high. When the Vpp becomes high, ion irradiation energy is increased and there is a possibility that the wafer is damaged.
As a method of avoiding this damage, there is a method in which the radio-frequency bias is not applied during the off-time of plasma. FIG. 2 shows an example thereof. The radio-frequency bias is also time-modulated in the same manner as the pulsed plasma and repeats on and of in synchronism with the pulsed plasma, so that the damage to the wafer during the off-time of the plasma can be avoided.